Acid–Base Status in Critically Ill Patients: Physicochemical vs. Traditional Approach

Summary

Ciabattoni et al. conducted a prospective observational study to evaluate the prevalence and progression of acid–base disturbances in mechanically ventilated ICU patients using traditional (standard base excess—SBE) and physicochemical (Stewart method) approaches. They found significant differences in diagnostic accuracy between the two methods, with poor agreement, highlighting the Stewart method’s potential superiority in capturing complex acid–base disorders, especially related to mechanical ventilation adjustments and fluid management.

Key Points:

1. Study Design and Population: This prospective observational study included 172 mechanically ventilated ICU patients to assess acid–base disturbances at admission and after 24 hours using both traditional and physicochemical methods.
2. Prevalence of Acid–Base Disorders: On ICU admission, acidemia was observed in 32% and alkalemia in 17% of patients. After 24 hours, acidemia decreased to 7%, while alkalemia increased to 28%.
3. Comparison of Diagnostic Methods: The occurrence rates of metabolic acidosis and alkalosis on ICU admission were similar using both SBE (24% acidosis, 16% alkalosis) and Stewart (35% acidosis, 23% alkalosis) methods, but their diagnostic agreement was poor.
4. Changes After 24 Hours: After 24 hours, the Stewart method identified metabolic acidosis in 36% and metabolic alkalosis in 16% of patients, significantly differing from the SBE method, which identified metabolic acidosis in 13% and alkalosis in 35%.
5. Factors Influencing Acid–Base Changes: Acidemia at admission correlated with higher PaCO2, lower SBE, and higher strong ion gap (SIG), while alkalemia was associated with lower albumin. After 24 hours, pH changes were linked to decreased PaCO2, albumin, and SIG without significant alterations in SIDa.
6. Role of Fluid Administration: Significant fluid administration from admission to day one contributed to changes in albumin and unmeasured anions, causing a shift toward alkalemia by diluting albumin and reducing SIG.
7. Urinary Acid–Base Variables: Urinary strong ion difference (SID) significantly increased after 24 hours due to increased retention of sodium and chloride alongside increased potassium excretion.
8. Renal Function Influence: Patients with impaired renal function (reduced glomerular filtration rate) had consistently lower pH, SBE, and higher SIG compared to those with normal renal function.
9. Predictive Accuracy of Methods: Standard Base Excess (SBE) demonstrated superior predictive accuracy for detecting metabolic acid–base disturbances compared to apparent strong ion difference (SIDa), particularly after adjusting for covariates like albumin and creatinine.
10. Clinical Implications: The study underscores the potential benefit of the Stewart method over the traditional approach in critically ill patients with complex and dynamic acid–base disorders, recommending its consideration for clinical implementation.

Conclusion

This study highlights the complexity of acid–base disturbances in mechanically ventilated ICU patients and suggests the physicochemical (Stewart) method offers better diagnostic capability than the traditional approach, particularly in capturing subtle and complex disturbances resulting from changes in mechanical ventilation and fluid management.

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